Documenting the Emergence of Xylazine Among People Who Use Drugs in Rhode Island: An Analysis of Dried Blood Spot Samples

Abstract

Background:

Xylazine has emerged in the unregulated drug supply and is associated with heavy sedation and severe skin wounds. This study uses dried blood spot (DBS) testing to examine xylazine exposure among a community-based sample of people who use drugs in Rhode Island.

Methods:

We conducted a secondary analysis of 234 people who use drugs enrolled in a clinical trial with DBS samples from October 2020 to December 2023. Participants completed interviews at multiple visits (baseline, 6, and 12 months); DBS samples were tested at 1 or more of those time points. Samples were tested for xylazine, fentanyl, and fentanyl analogs. We compared baseline sociodemographic and drug use characteristics of participants with and without detectable xylazine at any time point using descriptive statistics.

Results:

We found that 69 (29%) participants had detectable fentanyl at 1 or more time points, and among those, 27 (39%) had detectable xylazine. Detection of xylazine was associated with younger age, non-Hispanic white race, and prior-month homelessness (all P < .05). Among 397 DBS samples, 97 (24%) had detectable fentanyl and 32 (8%) had detectable xylazine. Among samples with detectable fentanyl, xylazine positivity was 24%, 21%, and 80% in years 2021, 2022, and 2023, respectively.

Conclusion:

DBS testing identified moderate levels of xylazine exposure among a sample of people who use drugs in Rhode Island during the observation period. Findings underscore the need for targeted xylazine education and harm reduction strategies focused on people who use fentanyl and unregulated opioids.

Keywords

xylazine dried blood spots people who use drugs drug testing substance use fentanyl

Highlights

12% of participants tested positive for xylazine using DBS testing.

Those testing positive for xylazine were more likely younger, non-Hispanic white, homeless.

Xylazine was strongly associated with fentanyl use and preference for fentanyl.

Among samples with detectable fentanyl, xylazine detection greatly increased in 2023.

Introduction

Over the past decade, continual changes in the unregulated drug supply have exacerbated drug-related morbidity and mortality.^1,2 Non-pharmaceutical fentanyl now dominates the unregulated opioid supply,³ and overdoses involving both stimulants and opioids (primarily fentanyl) characterize the “fourth wave” of the epidemic.^3-6 Xylazine, a veterinary tranquilizer, first emerged in the Puerto Rican drug market in 2001;⁷ however, more recently, xylazine has emerged as a novel substance detected alongside fentanyl in overdose postmortem toxicologies^8-11 and in public health–based surveillance of unregulated drug supplies in the continental United States.^12,13 First emerging in the Northeast, xylazine was present in 34% of overdose deaths in Philadelphia in 2022¹⁴ and approximately 9% of overdose deaths in Massachusetts in 2023.¹⁵ Between May 2022 and December 2023, a community-based drug-checking study known as testRI¹⁶ found xylazine in 44% of drug residue samples tested in Rhode Island; in every case, xylazine was detected alongside fentanyl.^17,18 Although the drug supply in the Northeastern United States shows a higher prevalence of xylazine than other regions,¹⁹ xylazine emerged on the West Coast in early 2023, with 29% of samples from a community-based drug-checking program in Los Angeles County showing concentrations of the substance by 2025.²⁰

Xylazine’s emergence is thought to be a result of the substance extending the “legs” (ie, duration) of drug effects and enhancing its euphoric effects.¹² However, other factors likely contribute to xylazine’s presence in the drug supply, including economic considerations such as cost, accessibility, and reduction in the availability of fentanyl.²¹ Research has documented numerous adverse experiences with xylazine, including heavy and prolonged sedation,^11,22,23 with increased risks of violence and victimization;²² blackouts and amnesia;^11,22 parasomnia;²² acute skin and soft tissue infections;^11,22,23 and symptoms of xylazine withdrawal, including anxiety and body aches.²³ Despite the documented adverse effects of xylazine exposure and numerous research publications about xylazine, scarce research has examined the emergence of xylazine in unregulated drug supplies in New England.

Several approaches have been employed to document trends in xylazine presence, including direct testing of drug samples (eg, community-based drug-checking programs),¹⁶ testing urine samples obtained from clinical settings,^24,25 and postmortem toxicology.¹⁰ Other chemical analyses could complement these approaches, particularly when drug-checking infrastructure or access to clinical care is limited.

The present study uses dried blood spot (DBS) testing, a collection method that affords greater sample stability, easier transport, and more feasible collection than traditional venipuncture-based blood sampling for subsequent centralized laboratory testing.^26,27 We examined the presence of xylazine in DBS samples that were collected among people who use drugs in Rhode Island from October 2020 through December 2023.²⁸ The objectives of this research were to: (1) identify participants’ sociodemographic and drug use pattern correlates of exposure to xylazine and (2) document changes in the prevalence of xylazine in DBS samples over the course of the observation period.

Methods

Setting and Data Collection

The parent study (ie, Rhode Island Prescription and Illicit Drug Study [RAPIDS]), described previously,²⁸ compared a fentanyl test strip and behavioral intervention to standard overdose education among people who use drugs; the primary aim of the parent study was to assess the efficacy of the intervention in reducing rates of overdose among people who use drugs. In this RAPIDS substudy, we collected DBS via capillary fingerstick to identify novel substances (eg, xylazine, emergent fentanyl analogs) that were not captured by the drug detection procedures used in the parent study. DBS was proposed as a “toxico-surveillance system” to characterize the changing drug supply over the study follow-up. Rhode Island residents were eligible if (a) they were aged 18 to 65 years; (b) they were able to complete interviews in English; (c) they were able to provide informed consent; and (d) they reported past 30-day substance use (ie, unregulated stimulants or opioids, prescription medications purchased extramedically, or any past-month injection drug use [IDU]). Participants were recruited using a combination of strategies, including internet-based advertising and marketing on state-wide public transit. Baseline and follow-up visits were completed from September 2020 to May 2024; DBS samples were collected from October 2020 through December 2023.

At baseline and 5 follow-up visits (1, 2, 3, 6, and 12 months post-baseline), participants completed a standardized, interviewer-administered questionnaire that elicited sociodemographic information, drug use patterns, and clinical characteristics. The primary outcome of the RAPIDS parent clinical trial was a composite measure of self-reported overdose in the previous month at 6 and 12-month follow-up visits; secondary outcomes included fatal and nonfatal overdose from postmortem investigation and hospitalization or emergency medical service utilization, respectively. At 3 study visits (ie, baseline, 6, and 12 months), fingerstick DBS samples were collected from consenting participants at fixed study sites. Thus, participants could have up to 3 DBS samples collected throughout the study period. Consent for DBS collection was obtained separately from consent for the main study assessments; therefore, only those participants who consented to DBS collection contributed DBS samples.

DBS Analysis

Collected DBS were dried overnight and stored at −80°C until processing. DBS samples with at least 1 visible blood spot were tested in the Brown University Health Toxicology laboratory for the presence of xylazine and fentanyl (Table S1) via Liquid Chromatography Quadrupole Time-of-Flight Mass Spectrometry (LC-QTOF-MS) on a Sciex X500R QTOF platform. Fentanyl products detected included analogs (para-fluorofentanyl, acetylfentanyl), metabolites (norfentanyl), or precursors (4-ANPP, NPP, phenethyl 4-ANPP, despropionyl para-fluorofentanyl) (Table S1). The DBS samples were analyzed using an untargeted acquisition method capable of detecting a wide range of substances; however, this investigation focused solely on xylazine and fentanyl because they were most relevant to the primary outcomes of the parent study (Table S1). DBS samples were routinely batched, stored, and sent to an off-site laboratory for analysis. Samples were then processed and analyzed at the collaborating laboratory, with results transmitted back to the study investigators approximately annually. At the consenting stage, participants were informed that the results would not be relayed back to them, given substantial delays in sample collection, transfer, and analysis. This secondary analysis includes RAPIDS participants who provided consent for the collection of DBS and provided ≥1 DBS sample (Figure S1). Table S2 presents baseline distributions of eligible participants and the broader RAPIDS cohort. We did not collect information on who was offered DBS testing, although it varied by survey administration site capabilities.

Measures

The primary outcome was whether participants had detectable xylazine at any time point (ie, baseline, 6, 12 months). Individual-level exposures assessed at baseline included participants’ self-reported sociodemographic characteristics (eg, age, sex at birth, gender identity, sexual orientation, race/ethnicity, homelessness status, monthly income, and history of incarceration), drug use patterns (regular use of unregulated drugs [ie, at least weekly in the prior month], IDU, sharing of syringes or supplies, drug selling history, overdose history), fentanyl-specific characteristics (regular fentanyl use, preference for fentanyl), and clinical characteristics (use of opioid agonist therapy [OAT], diagnosed depression, anxiety, and bipolar disorder).

Statistical Analysis

Results are stratified by the detection of xylazine in DBS at any time point (yes vs no). Groups were compared using Wilcoxon rank sum tests, Chi-squared tests, or Fisher’s exact tests as applicable. P-values ≤ .05 were considered statistically significant. A two-paneled histogram presents quarterly xylazine test positivity: One panel displays the proportion of samples with detectable xylazine, and the other shows the proportion among fentanyl-positive samples. Ethics approval was obtained from the Brown University Institutional Review Board.

Results

Sociodemographics

Among 503 RAPIDS participants, 264 (52%) were enrolled during the DBS sample collection period (October 2020 to December 2023) and consented to DBS testing (Figure S1). Almost half (234 participants, 47%) of the total sample provided 397 DBS samples over the 3 study visits: 121 (52%) participants provided 1 sample, 63 (27%) provided 2, and 50 (21%) provided 3 (Figure S1). The median age at enrollment was 44 years, 34% were assigned female at birth, 96% were cisgender, and 18% identified as non-Hispanic black (Table 1).

Table 1.

Individual Sociodemographic, Drug Use Patterns, and Clinical Characteristics of 234 People Who Use Drugs in Rhode Island Who Provided DBS Samples Stratified by any Detection of Xylazine from October 1, 2020, to December 31, 2023.

Characteristic	Overall N = 234 (%)	No xylazine detection n = 206 (88.0%)	Positive xylazine detection n = 28 (12.0%)	P-value^a
# of DBS samples, median (IQR)	1 (1)	1 (1)	2 (2)	.07
Sociodemographics
Age in years, median (IQR)	44.0 (18.8)	46.0 (18.8)	38.0 (9.3)	.01
Sex at birth
Male	154 (65.8)	136 (66.0)	18 (64.3)	.86
Female	80 (34.2)	70 (34.0)	10 (35.7)
Gender identity
Cisgender	224 (96.1)	196 (95.6)	28 (100)	.60
Other identity^b	9 (3.9)	9 (4.4)	0 (0)
Sexual orientation
Lesbian, gay, bisexual, or queer	33 (14.1)	30 (14.6)	3 (10.7)	.78
Straight or heterosexual	201 (85.9)	176 (85.4)	25 (89.3)
Race & ethnicity
Non-Hispanic white	133 (56.8)	109 (52.9)	24 (85.7)	.01
Non-Hispanic black^c	42 (17.9)	41 (19.9)	1 (3.6)
Non-Hispanic other/multiracial	20 (8.5)	19 (9.2)	1 (3.6)
Hispanic/Latine of any race	39 (16.7)	37 (18.0)	2 (7.1)
Homeless, prior month
Yes	98 (41.9)	81 (39.3)	17 (60.7)	.03
No	136 (58.1)	125 (60.7)	11 (39.3)
Monthly income
≤$500	90 (39.1)	73 (36.1)	17 (60.7)	.02
$501-1500	119 (51.7)	108 (53.5)	11 (39.3)
>$1500	21 (9.1)	21 (10.4)	0 (0)
History of incarceration, ever
Yes	184 (79.0)	164 (80.0)	20 (71.4)	.30
No	49 (21.0)	41 (20.0)	8 (28.6)
Drug use patterns
Injection drug use, ever	137 (58.5)	113 (54.9)	24 (85.7)	<.01
Share syringes/supplies, ever	93 (39.7)	77 (37.4)	16 (57.1)	.04
Drug selling, ever	156 (67.2)	137 (67.2)	19 (67.9)	.94
History of overdose, ever	137 (58.8)	119 (58.0)	18 (64.3)	.53
Regular use
Prescription opioids^d	74 (31.6)	63 (30.6)	11 (39.3)	.35
Benzodiazepines^d	67 (28.6)	54 (26.2)	13 (46.4)	.03
Crystal meth^d	34 (14.5)	22 (10.7)	12 (42.9)	<.01
Powder cocaine^d	63 (26.9)	53 (25.7)	10 (35.7)	.26
Crack cocaine^d	125 (53.4)	112 (54.4)	13 (46.4)	.43
Heroin^d	71 (30.3)	53 (25.7)	18 (64.3)	<.01
Fentanyl-specific characteristics
Regular use of fentanyl^e	56 (25.2)	35 (18.0)	21 (75.0)	<.01
Preference for fentanyl^f	35 (15.1)	23 (11.3)	12 (42.9)	<.01
Positive detection with DBS
Fentanyl analogs^g	28 (12.0)	8 (3.9)	20 (71.4)	<.01
Fentanyl metabolites^h	54 (23.1)	29 (14.1)	25 (89.3)	<.01
Fentanyl precursorsⁱ	36 (15.4)	13 (6.3)	23 (82.1)	<.01
Any of the above^j	69 (29.5)	42 (20.4)	27 (96.4)	<.01
Clinical characteristics
Opioid agonist therapy, current	69 (29.5)	56 (27.2)	13 (46.4)	.04
Any mental health diagnosis	176 (75.5)	153 (74.6)	23 (82.1)	.39
Depressive disorder	109 (46.8)	97 (47.3)	12 (42.9)	.66
Anxiety disorder	110 (47.2)	99 (48.3)	11 (39.3)	.37
Bipolar disorder	71 (30.5)	62 (30.2)	9 (32.1)	.84

Abbreviation: DBS, dried blood spot.

The Wilcoxon rank sum test was used to calculate P-values for continuous variables. Chi-squared test was used for categorical variables except when cell sizes were <5, when Fisher’s exact test was used to calculate P-values. A P-value of ≤.05 was considered statistically significant; values reaching statistical significance were bolded; ^bIncludes persons self-identified as transgender, genderqueer, “something else,” and those whose gender identity is nonconcurrent with sex at birth; ^cIncludes self-identified African, Haitian, and Cape Verdean ancestry; ^dIndicates at least 4 days of use in the prior 30 days; ^eIndicates “at least weekly” or “every day” use frequency in the past month; ^fCompares participants who answered “strongly agree” or “agree” to those who answered “neutral,” “disagree,” or “strongly disagree.”; ^gIncludes para-fluorofentanyl, acetylfentanyl; ^hIncludes norfentanyl; ⁱIncludes 4-ANPP, NPP, phenethyl 4-ANPP, despropionyl para-fluorofentanyl; ^jTen participants tested positive for fentanyl only. All other participants (n = 59) tested positive for fentanyl and 1 or more additional analog, metabolite, or precursor.

Twelve percent (n = 28) of participants had detectable xylazine through DBS testing during at least 1 study visit (Table 1). Among those with detectable xylazine during at least 1 study visit, 24 (86%) had 1 positive sample, 4 (14%) had 2, and none had 3.

Participants with detectable xylazine at any time point were 8 years younger at baseline than those who did not have detectable xylazine (P = .01). A significantly higher proportion of those with detectable xylazine were also non-Hispanic white, homeless in the prior month, and earned <$500 per month (all P < .05).

Drug Use Patterns

Over half (58%) of participants had ever injected drugs, and 40% had ever shared syringes/supplies. Both practices were more prevalent among those with positive xylazine results (P < .01 and P = .04, respectively). Those with detectable xylazine during the study period were also more likely to report regular use of benzodiazepines, crystal methamphetamine, heroin, and fentanyl (all P < .05).

Fentanyl-Specific Characteristics

At baseline, 25% of participants reported regular use of fentanyl in the prior month, and 15% reported preferring fentanyl over other drugs. Thirty percentage of participants tested positive for a fentanyl analog, metabolite, or precursor at least once during follow-up. All fentanyl-specific characteristics differed by the detection of xylazine in DBS, with those who tested positive for xylazine being more likely to use, prefer, and test positive for fentanyl (all P = .01). Among those with xylazine detected, 96% (n = 27) also had fentanyl detected.

Clinical Characteristics

Those engaged in OAT at baseline were significantly more likely to have xylazine detected (46% with detectable xylazine versus 27% without, P = .04). There were no significant differences in mental health diagnoses.

Detection of Xylazine Over Time

We present DBS sample results as a two-panel histogram, ranging from October 2020 through the end of December 2023, by quarter; Figure 1 shows results out of the total number of samples tested (Panel 1A, N = 397) and results among samples with fentanyl detected (Panel 1B, N = 97). Thirty-two of 397 DBS samples tested positive for xylazine over the study period, for an overall test positivity rate of 8%. Xylazine was detected in 6% and 5% of samples in 2021 and 2022, respectively, and increased precipitously to 21% of samples (12 of the 57 samples tested) in 2023 (Figure 1A). When restricting the sample to only those testing positive for fentanyl, xylazine positivity was 24%, 21%, and 80% in years 2021, 2022, and 2023, respectively (Figure 1B). All samples with fentanyl detected in the final 3 quarters of 2023 (n = 5) also contained xylazine. Lastly, we present the proportion of individuals with positive DBS sample results over time (Figure S2). There were 28 individuals who tested positive for xylazine at some point during study follow-up, with 9 participants testing positive in 2023.

Figure 1.

Two-panel histogram of xylazine detection out of the total number of samples tested using DBS (Figure 1A, N = 397, 32 samples with xylazine detected) and the total number of samples tested using DBS with fentanyl detection (Figure 1B, N = 97, 31 samples with both xylazine and fentanyl detected) from October 1, 2020, to December 31, 2023, among a sample of people who use drugs in Rhode Island. DBS, dried blood spot.

Discussion

Using DBS testing, this study documented moderate levels of xylazine exposure in a community-recruited sample of people who use drugs in RI. We found that 69 (29%) participants had detectable fentanyl, and among those, 27 (39%) had detectable xylazine. Participants exposed to xylazine tended to be younger, non-Hispanic white, homeless, and earn less money per month. They also tended to report IDU and sharing of supplies/syringes, among other drug use–related factors. There were some differences in self-reported regular use of fentanyl at baseline (25%) and positive detection of fentanyl at some point over the study period (30%); these differences may reflect unrecognized fentanyl exposure, differing recall periods, or variations in fentanyl use (eg, discontinued fentanyl use after baseline exposure and assessment). Finally, we identified an approximately four-fold increase in test positivity in 2023 compared with that in 2021 and 2022, contributing to the literature documenting the proliferation of xylazine in the drug supply in RI.

DBS was a feasible method of sample testing and could be added to the existing tools for chemical analysis in clinical and/or community-based settings, as it does not require venous access, phlebotomy, or access to medical or health care settings.²⁹ In light of the high prevalence of the co-occurrence of xylazine with fentanyl and the elevated presence of xylazine among those engaged in OAT, informative and educational harm reduction programming should be offered in settings that serve those who use unregulated opioids (eg, syringe service programs, OAT programs) in preventing or treating adverse reactions from xylazine (eg, skin wounds).

Our findings validate prior work in Rhode Island. Notably, a project called testRI collected drug samples from community members, used comprehensive toxicology testing via LC-QTOF-MS, and reported back to the community via plain language reports,^13,17 integration with existing public dashboards,³⁰ and peer-reviewed research.¹⁸ From May 2022 through January 2023, testRI documented that among samples with detectable fentanyl, 62% had detectable xylazine.¹⁸ Our study results show a very high co-occurrence of xylazine and fentanyl (aligned with testRI findings), with only 1 sample with detectable xylazine not also having detectable fentanyl, although fentanyl may have been present at levels below detection due to differences in the detection window for xylazine and fentanyl. Several factors may explain the differences between our results and those from testRI. Xylazine presence may be overrepresented in drug-checking service surveillance systems, as people may be more likely to submit samples that produced unusual effects. Recent pharmacokinetic research suggests that blood testing would adequately afford detection of xylazine alongside fentanyl,^31,32 so the difference in results may be due to sampling bias or differences in populations. For these reasons, it is imperative to monitor changes to the ever-shifting drug supply using comprehensive, multi-modality methods. Our research suggests that DBS may add to the robustness of existing drug-checking and xylazine surveillance systems in clinical and community-based settings.

This analysis is subject to some limitations. We did not quantify the concentration of xylazine in a given sample, which has been identified as an important area of ongoing and future research.²⁰ RAPIDS participants may differ from other populations of people who use drugs, and results from this study may not be generalizable to other contexts. Results may not be generalizable to non-English-speaking people in the United States, including those of Hispanic/Latino ethnicity. Also, the number of DBS samples tested in 2023 was fewer than those tested in previous years; it is unclear if this difference would skew results and, if so, in what direction. Lastly, those who agreed to DBS testing differed by some sociodemographic and substance use–related characteristics (less likely to be homeless and less likely to regularly use fentanyl) compared with the broader RAPIDS cohort, since only participants recruited through a fixed study site were eligible to provide DBS samples (Supplemental Table 1). Despite these limitations, our study contributes meaningfully to the growing body of evidence documenting the presence of xylazine in unregulated drug supplies and characterizing those most likely to test positive for xylazine.

Conclusions

DBS may be a useful tool to add to surveillance testing, especially in the context of an unregulated and rapidly changing drug market and fentanyl use. Given the heightened prevalence of co-occurring xylazine and fentanyl exposure among participants engaged with OAT in this study, our findings support expanding interventions that mitigate the effects of xylazine exposure (eg, drug checking, wound care) within the service settings that reach people who use fentanyl, such as syringe service programs and OAT programs.

Supplemental Material

sj-docx-1-saj-10.1177_29767342261450958 – Supplemental material for Documenting the Emergence of Xylazine Among People Who Use Drugs in Rhode Island: An Analysis of Dried Blood Apot Samples

Supplemental material, sj-docx-1-saj-10.1177_29767342261450958 for Documenting the Emergence of Xylazine Among People Who Use Drugs in Rhode Island: An Analysis of Dried Blood Apot Samples by Leah C. Shaw, Carolyn J. Park, Jason A. Scott, Mary C. Figgatt, Jacqueline E. Goldman, Brendan Jacka, Brandon D. L. Marshall, Adina Badea and Alexandria Macmadu in Substance Use & Addiction Journal

Footnotes

Acknowledgements

The authors would like to acknowledge the RAPIDS participants for their interest and participation in the study. Additionally, the authors would like to thank their RAPIDS researchers, students, and staff, both past and present, including Andrew Gould, Carolyn Park, Cathy Lenox, Esha Sridhar, Esther Moon, Evelyn Lai, Julia Trombley, Michael Tan, Roxxanne Newman, Sari Greene, Tania Lobo Paz, and Tayla Giguere.

ORCID iDs

Leah C. Shaw

Carolyn J. Park

Jason A. Scott

Mary C. Figgatt

Brandon D. L. Marshall

Alexandria Macmadu

Ethical Considerations

Ethics approval was obtained from the Brown University Institutional Review Board.

Consent to Participate

Verbal consent was obtained at the start of pre-screening for eligibility, and e-signed written informed consent was obtained at the start of baseline assessment for eligible participants. All participants were offered a copy of the e-signed informed consent via e-mail. All participants were given detailed explanations of their rights as human subjects.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The RAPIDS project is supported by the US National Institute on Drug Abuse [R01-DA0347975] and an administrative supplement [R01-DA047975-02S1]. Dr. Macmadu is supported by the Center for Biomedical Research Excellence (COBRE) on Opioids and Overdose at the Rhode Island Hospital, funded by the National Institute of General Medical Sciences [P20-GM125507], and the Collaborative Justice-Involved Research and Training (CJRT) Program on Substance Use and HIV at Brown University Health, funded by the National Institute on Drug Abuse [R25-DA037190].

Declaration of Conflicting Interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Data Availability Statement

Reasonable requests for data will be considered by the corresponding author. De-identified data may be provided as long as there are continued protections for anonymity.

Supplemental Material

Supplemental material for this article is available online at the SAJ website .

References

CDC. Understanding the Opioid Overdose Epidemic [Internet]. Overdose Prevention. Updated 2025. Accessed August 19, 2025. https://www.cdc.gov/overdose-prevention/about/understanding-the-opioid-overdose-epidemic.html

Cerdá

Krawczyk

Keyes

The future of the united states overdose crisis: challenges and opportunities. Milbank Q. 2023;101(Suppl 1):478-506.

Mattson

Tanz

Quinn

Kariisa

Patel

Davis

Trends and geographic patterns in drug and synthetic opioid overdose deaths—United States, 2013-2019. Morb Mortal Wkly Rep. 2021;70:202-207.

Townsend

Kline

Rivera-Aguirre

, et al. Racial/ethnic and geographic trends in combined stimulant/opioid overdoses, 2007-2019. Am J Epidemiol. 2022;191(4):599-612.

Ciccarone

The rise of illicit fentanyls, stimulants and the fourth wave of the opioid overdose crisis. Curr Opin Psychiatry. 2021;34(4):344-350.

Jenkins

RA.

The fourth wave of the US opioid epidemic and its implications for the rural US: a federal perspective. Prev Med. 2021;152(Pt 2):106541.

Torruella

RA.

Xylazine (veterinary sedative) use in Puerto Rico. Subst Abuse Treat Prev Policy. 2011;6:7.

Johnson

Pizzicato

Johnson

Viner

Increasing presence of xylazine in heroin and/or fentanyl deaths, Philadelphia, Pennsylvania, 2010-2019. Inj Prev J Int Soc Child Adolesc Inj Prev. 2021;27(4):395-398.

Chhabra

Mir

Hua

, et al. Notes from the field: xylazine-related deaths—Cook County, Illinois, 2017-2021. MMWR Morb Mortal Wkly Rep. 2022;71(13):503-504.

10.

Kariisa

Patel

Smith

Bitting

Notes from the field: xylazine detection and involvement in drug overdose deaths-United States, 2019. MMWR Morb Mortal Wkly Rep. 2021;70(37):1300-1302.

11.

Friedman

Montero

Bourgois

, et al. Xylazine spreads across the US: a growing component of the increasingly synthetic and polysubstance overdose crisis. Drug Alcohol Depend. 2022;233:109380.

12.

Bowles

McDonald

Maghsoudi

Xylazine detected in unregulated opioids and drug administration equipment in Toronto, Canada: clinical and social implications. HRJ. 2021;18:104. https://harmreductionjournal.biomedcentral.com/articles/10.1186/s12954-021-00546-9

13.

Shihipar

. testRI releases preliminary results of drug surveillance study. People, Place & Health Collective. Medium. Updated 2022. Accessed July 2, 2025. https://medium.com/pphc/testri-releases-preliminary-results-of-drug-surveillance-study-ac39599daee4

14.

Unintentional Drug Overdose Fatalities in Philadelphia, 2022 . Department of Public Health. Updated September, 2023. https://www.phila.gov/media/20231002090544/CHARTv8e3.pdf

15.

Massachusetts Department of Public Health. Data Brief: Opioid-Related Overdose Deaths among Massachusetts Residents [Internet]. Department of Public Health. Updated June, 2024. https://www.mass.gov/doc/opioid-related-overdose-deaths-among-ma-residents-june-2024-0/download

16.

What is testRI? [Internet]. test RI Research Study. Accessed July 2, 2024. https://sites.brown.edu/testri/

17.

Shihipar

testRI Final Report & Findings [Internet]. People, Place & Health Collective. Updated 2024. Accessed August 6, 2025. https://medium.com/pphc/testri-final-report-findings-2da20b680ebd

18.

Collins

Wightman

Macon

, et al. Comprehensive testing and rapid dissemination of local drug supply surveillance data in Rhode Island. Int J Drug Policy. 2023;118:104118.

19.

El-Bassel

Shoptaw

Skalland

, et al. Prevalence of illicit drug detection in 5 US cities among out-of-treatment people who inject drugs. JAMA Netw Open. 2026;9(2):e2555882.

20.

Friedman

Molina

Koncsol

, et al. Xylazine prevalence and concentration in the Los Angeles fentanyl market, 2023-2025. Drug Alcohol Depend Rep. 2025;16:100364.

21.

D’Orazio

Nelson

Perrone

Wightman

Haroz

Xylazine adulteration of the heroin–fentanyl drug supply: a narrative review. Ann Intern Med. 2008;176(10):1370-1376. https://www.acpjournals.org/doi/10.7326/M23-2001

22.

Reed

Imperato

Bowles

Salcedo

Guth

Rising

KL.

Perspectives of people in Philadelphia who use fentanyl/heroin adulterated with the animal tranquilizer xylazine; Making a case for xylazine test strips. Drug Alcohol Depend Rep. 2022;4:100074.

23.

Spadaro

Connor

Lakamana

, et al. Self-reported xylazine experiences: a mixed methods study of reddit subscribers. medRxiv. 2023; 2023.03.13.23287215.

24.

Holt

Schwope

Schrecker

Heltsley

Widespread distribution of xylazine detected throughout the United States in healthcare patient samples. J Addict Med. 2023;17(4):468-470.

25.

Hauschild

Warp

Tookes

, et al. Prevalence of xylazine among people who inject drugs seeking medical care at a syringe services program clinic: Miami, Florida, 2023. Drug Alcohol Depend Rep. 2023;9:100209.

26.

Frederick

DL.

Toxicology testing in alternative specimen matrices. Clin Lab Med. 2012;32(3):467-492.

27.

Zakaria

Allen

Koplin

Roche

Greaves

RF.

Advantages and challenges of dried blood spot analysis by mass spectrometry across the total testing process. EJIFCC. 2016;27(4):288-317.

28.

Jacka

Goldman

Yedinak

, et al. A randomized clinical trial of a theory-based fentanyl overdose education and fentanyl test strip distribution intervention to reduce rates of opioid overdose: study protocol for a randomized controlled trial. Trials. 2020;21:976.

29.

Kania

Bekalé

Nagot

, et al. Combining rapid diagnostic tests and dried blood spot assays for point-of-care testing of human immunodeficiency virus, hepatitis B and hepatitis C infections in Burkina Faso, West Africa. Clin Microbiol Infect Off Publ Eur Soc Clin Microbiol Infect Dis. 2013;19(12):E533-E541.

30.

Local Drug Supply POR, RI Department of Health (RIDOH), Behavioral Healthcare, Developmental Disabilities and Hospitals (BHDDH), Brown University (brown edu), Rhode Island Gov (RI GOV). Local Drug Supply—Prevent Overdose RI [Internet]. Prevent Overdose RI. Accessed October 10, 2023. https://preventoverdoseri.org/local-drug-supply/

31.

Bird

Huhn

Dunn

KE.

Fentanyl absorption, distribution, metabolism, and excretion (ADME): narrative review and clinical significance related to illicitly-manufactured fentanyl. J Addict Med. 2023;17(5):503-508.

32.

Lin

Farnsworth

Azimi

Liss

Mullins

Crews

BO.

Xylazine pharmacokinetics in patients testing positive for fentanyl and xylazine. Clin Chem. 2025;71(2):266-273.

Supplementary Material

Please find the following supplemental material available below.

For Open Access articles published under a Creative Commons License, all supplemental material carries the same license as the article it is associated with.

For non-Open Access articles published, all supplemental material carries a non-exclusive license, and permission requests for re-use of supplemental material or any part of supplemental material shall be sent directly to the copyright owner as specified in the copyright notice associated with the article.

1.90 MB

0.00 MB